This invention relates to an image reading device which includes a plurality of image sensors arranged in a widthwise direction of a document, each image sensor including photoelectric conversion elements such as phototransistors arranged in line, so as to scan sequentially from one lateral end to the other lateral end in the widthwise direction to read a document image, and outputs read data to such apparatus as image forming apparatus.
Conventionally, in an image reading device for use in an image forming apparatus such as a facsimile machine, there is arranged in a widthwise direction of a document an image sensor including a photoelectric array in which photoelectric conversion elements such as photodiodes are arranged in line at a specified interval. The image reading device reads a document image by causing the image sensor to scan in the widthwise direction (hereinafter referred to as a main scanning) and by moving the image sensor or document relatively to scan in a lengthwise direction of the document (hereinafter referred to as a sub-scanning). For example, the document is moved in a direction opposite to a sub-scanning direction as shown in FIG. 12A, while the image sensor is moved in the same direction as the sub-scanning direction as shown in FIG. 12B.
There is a limit in the number of photoelectric conversion elements mounted on a single image sensor to keep a desired reading speed. Accordingly, the number of photoelectric conversion elements and the length of the image sensor become insufficient in the case where the document width is large. Thus, in this case, a plurality of image sensors are arranged side by side according to the document size, and is caused to scan in the same direction so as to read the document image. At this time, a microscope is used to arrange the image sensors lest they should slip relative to one another in the sub-scanning direction.
In this way, the array of photoelectric conversion elements sequentially scan so as to read the document image. Accordingly, there is a time difference between a reading timing of the first photoelectric conversion element and that of the last one. On the other hand, as described above, the image sensor and the document are moved relatively in the lengthwise direction of the document while the photoelectric array is being scanned. Accordingly, a reading track is inclined with respect to a main scanning direction. For example, as shown in FIG. 13, in the case where a straight line S on a document G is read by image sensors 1 to 4, a reproduced image of the straight line S becomes discontinuous due to a difference in scanning time between two successively operated image sensors.
More specifically, in the case of converting the light received analog signal from the photoelectric array into a digital signal of multiple bits, e.g., eight bits, as shown in FIG. 13, the reproduced image is liable to have irregularities at respective portions corresponding to boundaries between the juxtaposed image sensors 1 to 4.
Also, in the case of converting the light received analog signal into a digital signal of one bit, the reproduced image is liable to have discontinuities due to the fact that some photoelectric conversion elements on a reading completion end portion of the array do not reach the threshold level, and cannot read out the straight line G, and consequently generate white image data. Consequently, the document image cannot be reproduced accurately, and the appearance of the reproduced image is deteriorated.
As a measure against this problem, Japanese Unexamined Patent Publication No. 2-202265 discloses an image reading device in which adjacent image sensors are arranged such that main scanning directions thereof are alternately opposite to each other so as to correspond to the reading timings at boundaries between two successively operated image sensors, to thereby eliminate the aforementioned discontinuity in the output data from the image reading device.
In the image reading device disclosed in the above publication, however, since the main scanning directions of adjacent image sensors are alternately opposite to each other, wires are connected to one end of the respective image sensors alternately at opposite positions, thereby complicating the wiring. In addition, the image sensors are required to be arranged after discriminating the main scanning directions thereof, causing a reduction in the working efficiency. Further, when the reading tracks of the respective image sensors are connected, the reproduced image is bent, thereby deteriorating the reproducibility and appearance.
In order to solve this bending in the reproduced image, this prior art document discloses that the image sensors are arranged or inclined along the inclination of the reading tracks of the respective image sensors. However, this arrangement necessitates adjustment of the inclination of the respective image sensors, thus costing time and labor. Also, an adjusting operation requires skill since the inclination is adjusted by an extremely fine angle. In the arrangement of image sensors in the bending form, further, there is remarkably great space in the boundary between the image sensors, i.e., between the scanning completion end photoelectric conversion element of one image sensor and the scanning start photoelectric conversion element of the next image sensor. Consequently, this arrangement involves unreadable portions, and results in discontinuities in the reproduced image.